A group of Labour MPs has demanded that thousands of grassroots members should be consulted over Brexit amid further confusion on the party's approach.
Pro-EU parliamentarians and campaigners have written to the party's ruling body to call for a new commission that would thrash out Labour's stance on leaving the European Union.
In a letter to NEC chair Andy Kerr, the group, led by former shadow health secretary Heidi Alexander, said it was "deeply disappointed" that there was no commission to look at Brexit among the eight policy groups the NEC established after the election.
The move will heap further pressure on Jeremy Corbyn to distance himself from the Government on Brexit, whose plan to leave the single market and the customs union was branded "reckless and cavalier" by the letter's signatories.
It comes amid fresh confusion over the Labour leader's Brexit views, as the party was forced to deny suggestions Mr Corbyn had told the EU's chief Brexit negotiator Michel Barnier that he would be open to staying in the customs union.
The letter, co-ordinated by Ms Alexander and Wirral South MP Alison McGovern, said: "Brexit is the single biggest issue facing our country and our members deserve a say.
"This should be a deliberative process, such as that which has historically been enabled by our policy commissions, and should culminate in a full debate at party conference later this year. This is not a fringe issue and should not be treated as such.
"With the European Union demanding clarity from the Government on its desired for the long term arrangements between the UK and the EU before the end of March, there is an urgent and pressing need for the Labour Party to set out an alternative approach."
Labour has managed to maintain a broad position on the future relationship with the EU but pressure is mounting on party leadership to take a clearer stance.
The leaders of four opposition parties wrote to Mr Corbyn urging him to “rethink” his stance on Brexit and work with them to keep the UK in the EU single market and customs union.
Carwyn Jones, the Labour Welsh First Minister, recently told The Independent he expected Mr Corbyn to announce a shift in policy in favour of staying in the customs union “within months” and warned that leaving the bloc would be “economically daft”.
A Labour Party spokesperson said: "The NPF considers policy issues through eight policy commissions; that includes Britain's exit from the EU, which is the responsibility of the International commission. The Commission and wider NPF are looking at the issue this year in meetings, evidence sessions and by considering all submissions received."
Meanwhile, a leaked EU memo of a meeting between the Labour leader and Mr Barnier, suggested that Mr Corbyn had said he was open to the idea of remaining in the existing customs union after Brexit, according to The Telegraph.
Labour's current policy favours a form of customs union but it has committed to leaving the existing version, which allows tariff-free trade in goods between EU member states.
A spokesman for the Labour leader said: "Jeremy did not say he was open to staying in the customs union. He said that a customs union was a viable end point.
"We have been clear all the way through that you can't be in the customs union if you are not in the EU."
